The present invention relates to a cathode ray tube apparatus, or more in particular, to a cathode ray tube apparatus comprising a deflection yoke capable of reducing the deflection power and the leakage magnetic field effectively.
Generally, the cathode ray tube apparatus comprises a vacuum envelope of glass and a deflection yoke forming a deflection magnetic field for deflecting the electron beams. The vacuum envelope includes a rectangular faceplate, a cylindrical neck portion and a funnel portion for coupling the faceplate and the neck portion to each other. The deflection yoke is mounted over the portion extending from the. neck portion to the yoke portion in the funnel portion.
In the cathode ray tube apparatus having this construction, the deflection power supplied to the deflection yoke is the main power consumed in the apparatus. In recent years, in order to satisfy the requirement for high brightness and high definition of the cathode ray tube apparatus, the trend is toward an even more increased deflection power. For the power consumption of the cathode ray tube apparatus to be reduced, however, the deflection power is required to be decreased. Also, with this cathode ray tube apparatus, it is necessary to reduce the magnetic field leaking from the deflection yoke out of the cathode ray tube apparatus.
Generally, for reducing the deflection power and the leakage magnetic field, the outer diameters of the neck portion and the yoke portion are desirably reduced. With this structure, the operating space of the deflection magnetic field is reduced and the operating efficiency of the deflection magnetic field exerted on the electron beams is improved.
In the conventional cathode ray tube apparatus, however, the electron beams pass in proximity to the inner surface of the yoke portion. If the outer diameters of the neck portion and the yoke portion are reduced, therefore, the electron beam having a large deflection angle, that is, having an electron beam trajectory at a large angle to the tube axis impinges on the inner wall of the yoke portion. Such an electron beam fails to reach on the phosphor screen and causes a display failure. In the cathode ray tube apparatus having this construction, it is difficult to reduce the deflection power and the leakage magnetic field by reducing the outer diameters of the neck portion and the yoke portion.
U.S. Pat. No. 3,731,129 discloses a cathode ray tube in which the shape of a section perpendicular to the tube axis of the yoke portion changes progressively from a circle to a rectangle starting with the neck portion toward the faceplate. With this pyramidal yoke portion, the outer diameters of the yoke portion can be reduced without being impinged the electron beam on the inner wall of the yoke portion. Also, with this structure, the deflection magnetic field acts on the electron beam with a comparatively high efficiency.
In the cathode ray tube apparatus of this configuration, however, the side surfaces of the yoke portion flatten more and the environmental pressure resistance of the yoke portion of the envelope is reduced more, the higher the rectangularity of the yoke portion. Thus the safety is adversely affected.
Recently, a flat display unit with a flat outer surface of the faceplate has found an application. In the flat display unit with an outer surface having a radius of curvature at least twice the effective diagonal length of the phosphor screen (the faceplate is completely flat when the radius of curvature is infinitely large), however, the environmental pressure resistance of the faceplate is low and the yoke portion, if pyramidal, decreases also in the environmental pressure resistance, thereby making it difficult to secure a mechanical strength required of the vacuum envelope as a whole for safety. The strength of the vacuum envelope, that is, the environmental pressure resistance and the mechanical strength thereof combined will hereinafter be collectively called the bulb strength.
The two requirements described above, that is, a rectangular section of the yoke portion in order to sufficiently reduce the deflection power and the leakage magnetic field on the one hand and a sufficient bulb strength even with a rectangular section of the yoke portion on the other, cannot be met at the same time by the conventional cathode ray tube apparatus. It is especially difficult for the cathode ray tube apparatus with a flat display unit to reduce the deflection power and the leakage magnetic field and a sufficient bulb strength at the same time.